The utility model relates to an instantaneous heater comprising a heater main body. The heater main body comprises a water pump, a heating core water liner, a water feeding mouth, a control component and a main body casing which is arranged at the outer part; the heating core water liner comprises a core, at least a heat diffusion part, at least a heating body and a casing, wherein the core is provided with a water feeding level, a water outlet level and a channel connecting the water feeding level and the water outlet level, one surface of the heat diffusion part is connected with the core, and the other surface of the heat diffusion part is connected with the heating body; the water feeding mouth is provided with a water inlet and a water outlet, the water inlet is connected with the water outlet level of the core, and a temporary storage space and a steam chamber are arranged between the water inlet and the water outlet; and the main body casing is provided with a first control panel which is provided with a control button, and the control button controls the input current of the water pump. The instantaneous heater has stable effluent water effect and stable effluent water temperature, and can reach the preset temperature value; and simultaneously a complicated electronic control system does not need to be arranged, and the cost is reduced.